Inorganic materials such as quartz glass and multi-component glass characterized by a small optical propagation loss and a wide transmission band have been widely used as a base material for optical components or optical fibers. In recent years, polymer materials have been also developed. The polymer materials are excellent in workability and cost as compared with the inorganic materials, so that they receive attention as an optical waveguide material.
For example, a slab optical waveguide has been developed, that has a core-clad structure composed of a core made of a polymer having a high transparency such as polymethyl methacrylate (PMMA) and polystyrene, and a clad made of a polymer having a lower refractive index than the core material. Further, there has been put into practice a slab optical waveguide with low losses using a polyimide that is a transparent polymer having high heat resistance (for example, see Patent Document 1).
An optical waveguide made of the polymer materials is flexible, so that it is expected to be coupled with a semiconductor laser, a quartz optical fiber or the like, maintaining low losses without damaging the edges thereof (for example, see Patent Document 2).
Further, because the optical waveguide made of the polymer materials has flexibility, it is expected to be used similarly to a flexible electric circuit board used for electric circuits. The flexible electric circuit board is, for example, disposed in a manner that it bridges across two mother boards connected with each other with the help of a hinge as is found in cellular phones and the like. At the hinge, the flexible electric circuit board is rolled up into a bar or hollow with a curvature radius corresponding to the size of the hinge. Then, the flexible electric circuit board is covered with a protective hood or the like having a size slightly larger than the curvature radius.
In recent years, cellular phones are required to have high speed transmission performance, space saving capability and the like, so that the circuit board is folded at the hinge at a small folding radius (around 2 mm). Therefore, in the flexible electric circuit board, there have actually arisen problems such as noise troubles or picture quality degradation. Among the countermeasures to meet the problems, an optical wiring can be selected in place of conventional electric wirings. A flexible optical waveguide film may be one of the candidates for the optical wiring.
In the case where an electric wiring is required along with an optical wiring at the position bridging across the hinge so as to, for example, supply electric power to one of the mother boards, it is suggested to connect each wiring separately to the mother boards. Alternatively, it is suggested to use an electrical and optical hybrid circuit film, which has an electric wiring layer formed on an optical waveguide film. The electrical and optical hybrid circuit film may meet the requests for space saving, thin film fabrication, and miniaturization. However, the total thickness of a unified electrical and optical hybrid circuit film formed by laminating an optical waveguide film and a flexible printed circuit board becomes large (for example, over 150 μm), thereby the durability for folding possibly becomes lowered.
In order to make the thickness of the optical waveguide film so small as to increase the durability for folding, it is suggested to reduce the core size of the optical waveguide film. However, as the core size of the optical waveguide film becomes small, the allowance for positional shift with respect to the other optical components becomes small, leading to decrease in the optical coupling efficiency. For example, when the optical waveguide film and the other optical components are aligned for optical coupling with each other, the core diameter at an optical input is required to be around 100 μm to 150 μm at present. The optical waveguide film has an additional thickness of about 30 μm besides the core diameter. The optical waveguide film having such thickness possibly not only has optical losses, but also causes failures such as the rupture of the optical guide at a folded portion. Further, when the optical waveguide film and the electrical wiring film are unified by lamination, the folded portion gets still thicker by 10 μm to 50 μm, thereby the durability for folding becomes still more lowered.
On the other hand, a transfer process is known as one of the methods for producing the polymer optical waveguide film. The transfer process includes the steps of: coating a resin composing a clad to a mold having an projection corresponding to a core to obtain a clad film on which a groove is formed through transferring the projection, inside which the core is going to be formed; filling a resin composing the core into the groove on the clad film; peeling off the clad film from the mold; and further coating a resin composing a clad on the core in the groove to form an optical waveguide provided with the core embedded in a clad. In the transfer process, a core made of a polyimide is formed by coating and drying a resin precursor (polyamide acid) solution containing a number of solvents. When the solvents are evaporated, the size of the core is sometimes largely reduced. Considering better alignment between the optical waveguide and an emitting element, it is desirable that the core diameter is large. Therefore, in some cases, it is not desirable that the size of the core is reduced in the process.
Further, a process of machining a core with a dicing saw has been proposed (for example, see Patent Document 3) as one of the methods for producing the polymer optical waveguide film. Two grooves are formed using a dicing saw on a laminate including a layer composed of a core material and formed on a clad layer, and then a part of the layer composed of the core material is removed to form a core. After that, a clad material is coated over the laminate to fill in the grooves with the clad material. In accordance with this process, the thickness of the core can be increased, but the grooves are fully embedded with the resin. Therefore, for example, in order to make the core thickness be 50 μm through filling in the grooves on a film corresponding to the core, the total thickness of the film is required to be 100 μm or more. Such a thick film has a poor durability for folding and is also weak against twisting when the thick film is used while it is folded or rolled around a hinge in a device. To the contrary, when the total thickness of the film is reduced, the film becomes difficult to handle after the grooves are machined, and the film is sometimes warped or deformed when the clad material (resin) is embedded in the grooves.
Patent Document 1: Japanese Patent Laid-Open Publication No. H04-9807
Patent Document 2: Japanese Patent Laid-Open Publication No. 2002-318318
Patent Document 3: Japanese Patent Laid-Open Publication No. H08-286064